1. Technical Field
The present invention relates to a method of controlling a stepping motor, an apparatus for controlling a stepping motor, and a printer.
2. Related Art
In the related art, in ink jet printers for performing printing on, for example, printing sheets, a stepping motor has been used as a feed motor for rotating a feed roller to transport the printing sheets (for example, see Patent Document 1 and Patent Document 2). A stepping motor used in the ink jet printers disclosed in Patent Document 1 and Patent Document 2 is a 2-phase stepping motor including A-phase and B-phase magnetic pole sets arranged with a phase difference therebetween corresponding to an electrical angle of 90° and driving coils wound around the two magnetic pole sets. In addition, ink jet printers using DC (direct current) motors as feed motors for transporting printing sheets have been proposed (for example, see Patent Document 3).
As disclosed in Patent Document 3, the printer using the DC motor as the feed motor is generally provided with a rotary encoder for controlling a process of transporting printing sheets. Therefore, the printer using the DC motor as the feed motor can perform printing with high resolution by using the rotary encoder, as compared to printers using stepping motors as the feed motors (that is, the printer can transport printing sheets with high resolution). In contrast, the printer using the DC motor needs to have the rotary encoder, which causes the manufacturing costs of the printer including the DC motor to be higher than those of the printer including the stepping motor. Therefore, in general, the stepping motor is used as a feed motor of a relatively inexpensive printer not requiring high-resolution printing.
Patent Document 1: JP-A-2004-56991
Patent Document 2: JP-A-10-323090
Patent Document 3: JP-A-2001-232882
In recent years, demands for inexpensive and high-resolution printers have increased on the market. However, in the stepping motor, a step angle is determined by the number of magnetic poles (pole teeth), and resolution is determined by the step angle. Therefore, there are limitations in improving the resolution of the stepping motor by a change in the mechanical structure of the stepping motor. In addition, it is possible to increase the gear ratio of gears connected to the stepping motor and the feed roller to improve the resolution of the stepping motor, which causes the transport speed of printing sheets to be lowered, resulting in a low printing speed.
The 2-phase stepping motor can be driven in any one of the following excitation modes to transport printing sheets with high resolution: a 1-2-phase excitation mode that rotates a rotor at a step angle of 45°, which is an electrical angle, in theory; a W1-2-phase excitation mode that rotates a rotor at a step angle of 22.5°, which is an electrical angle, in theory; and a 2W1-2-phase excitation mode that rotates a rotor at a step angle of 11.25°, which is an electrical angle, in theory.
However, the inventors' studies show that, when a general stepping motor driving circuit is used to driving a stepping motor, for example, as shown in FIG. 18, although the theoretical rotational position of the rotor is θ1 in electrical angle, the actual rotational position of the rotor specified by a current actually supplied to the driving coil wound around the A-phase magnetic pole set and a current actually supplied to the driving coil wound around the B-phase magnetic pole set is θ2 in electrical angle. That is, there is a place where a resultant vector V 20 of a current value C100A that is actually supplied to the A-phase magnetic pole set and a current value C100B that is actually supplied to the driving coil wound around the B-phase magnetic pole set is not equal to a theoretical resultant vector V10.
In addition to the driving characteristics of the stepping motor, the printer provided with the stepping motor is affected by mechanical loads, such as loads between gears connecting the stepping motor and the feed roller, a transport load of the printing sheet, and detent torque, which is residual torque generated when no current is supplied to the driving coils. Therefore, the inventors' studies show that, during the driving of the stepping motor in the 1-2-phase excitation mode, the W1-2-phase excitation mode, or the 2W1-2-phase excitation mode, the stop accuracy of the rotor is considerably lowered when the rotor stops at positions other than the stop position of the rotor in the 2-phase excitation mode of the stepping motor. That is, when the rotor stops at positions other than the position corresponding to an electrical angle of 45°, 135°, 225°, or 315° shown in FIG. 18, the stop accuracy of the rotor is considerably lowered. As a result, it is difficult to perform printing with high resolution.
The inventors' studies prove that, even when the rotor stops at positions other than the stop position of the rotor in the 2-phase excitation mode of the stepping motor, a predetermined control process makes it possible to prevent the stop accuracy of the rotor from being lowered.